Evolution of NITrogen BUFFERing capacity of land water interfaces along hydrosystems of different age (NITBUFFER)

河漫滩、河岸和河内带是河流生态系统中能量和物质转移的关键调节器。更具体地说，河流景观中的陆水界面，如河岸森林或草甸、湿地、沙砾坝，在那里发生物理沉积和生物活动；作为生物地球化学热点，特别是氮循环。这些界面也代表了功能保留区，即缓冲区（Haycock等人，1997年），控制和维持河流水质（Sabater等人，2003年）。经验证据表明，水-基质界面面积（即水-沉积物或湿地-高地接触长度）与河流生态系统氮保持和吸收效率呈正相关。然而，量化水陆界面对氮循环及其在流域缓冲能力的重要性的努力在很大程度上是不成功的，因为；1)这些界面在原位研究的尺度与它们在更大尺度上的能力的外推之间的差异；2)在世界大部分地区，人类活动导致河流生态系统破碎化和栖息地破坏，破坏了这些生态系统的结构和功能，因此很难准确地破译这些陆水界面的作用。然而，河岸带现在已被公认为一种工具，既可以保护河流水质，防止扩散污染，又可以促进河流栖息地的再生。然而，我们对这种恢复对这些陆地-水界面氮缓冲能力的影响缺乏基本的了解。此外，基于河岸带恢复的河流恢复轨迹尚不完全清楚，主要是因为我们不知道水文地貌和微生物过程相互作用的自然机制以及这些相互作用实现氮缓冲能力的时间框架。本研究的总体目标是利用阿拉斯加东南部冰川湾在过去250年冰川消退后形成的不同年龄和复杂程度的流域作为海洋气候下的天然原位实验室，分析水文地貌发育和微生物过程控制水系陆水界面氮缓冲能力的共同演化。这项研究将首次深入了解陆地水界面形成和发展的自然时间框架，以及它们对河流集水区氮调节的影响。在冰川湾选择海洋性气候下的原始水系是合适的，因为：i)在植被和河流动物演替方面，存在一个有充分记录的时间序列；Ii)时空策略可用于量化陆地水结构和功能的变化率；iii)地球化学示踪剂在这一原始环境中不受过去和现在人类遗产的影响；iv)选择大小相似（约10平方公里）的流域，可以确定景观结构和排列对待确定的氮通量的重要性。这个天然的原位实验室允许制定和测试与陆地水界面形成和氮缓冲能力发展有关的假设。调节陆地水界面氮循环的微生物过程的原位速率测量、有机质特征和浓度以及地球化学指标将提供关于含氮陆地水缓冲区演化速率的宝贵数据。这些数据对于了解冰川消融对温室气体排放（N2O、CO2、CH4）的影响以及氮缓冲能力随时间发展的机制至关重要。

Floodplain, riparian and in-stream zones are key regulators of energy and matter transfer in river ecosystems. More specifically, terrestrial-aquatic interfaces in riverine landscapes, e.g. riparian forest or meadow, wetlands, gravel bars, where physical sedimentation and biological activities occur; act as biogeochemical hot spots, particularly for nitrogen cycling. These interfaces also represent functional retention areas, i.e. buffer zones (Haycock et al. 1997) which control and maintain river water quality (Sabater et al. 2003). Empirical evidence has shown that the area of water-substrate interface (i.e. water-sediment or wetland-upland length of contact) is positively correlated to the efficiency of nitrogen retention and uptake in river ecosystems. Nevertheless, efforts to quantify the importance of land-water interfaces on nitrogen cycling and their buffering capacity in drainage basins have largely been unsuccessful due to; i) the discrepancy between the scales at which these interfaces have been studied in situ and the extrapolation of their capabilities at larger scales, and ii) anthropogenic activities which have led to river ecosystem fragmentation and habitat destruction in most parts of the world, disrupting the structure and function of these lotic ecosystems, such that it is difficult to accurately decipher the role of these land-water interfaces. However, riparian zones are now well recognised as a tool to allow both protection of river quality against diffuse pollution and to promote the regeneration of stream habitats. Yet, we lack a fundamental understanding of the consequences of this restoration on the nitrogen buffering capacities of these land water interfaces. Moreover the trajectory of stream restoration based on the rehabilitation of riparian zones is not fully understood, mostly because we do not know the natural mechanisms by which hydrogeomorphological and microbial processes interact and the timeframe of these interactions to achieve nitrogen buffering capacities. The overall goal of the proposed research is to use watersheds of different ages and complexity that have developed during the last 250 years following deglaciation in Glacier Bay, southeast Alaska as a natural in-situ laboratory under maritime climate to analyse the co-evolution of hydrogeomorphic development and microbial processes controlling nitrogen buffering capacity in hydrosystems' land water interfaces. This research will provide the first insights into the natural timeframe of land water interface formation and development, and their consequences on nitrogen regulation in stream catchments. The choice of pristine hydrosystems under Maritime climate in Glacier Bay is appropriate due to: i) the existence of a well documented chronosequence, both in terms of vegetation and stream fauna successions; ii) space-for-time strategies can be used to quantify rates of change of land water structure and functions; iii) geochemical tracers are not influenced by past and present human legacy in this pristine context, and iv) the selection of drainage basins of similar size (ca 10 km2) permits to determine the importance of landscape structure and arrangement on N fluxes to be determined. This natural in-situ laboratory allows the formulation and testing of hypotheses related to the formation of the land water interface and the development of their nitrogen buffering capacity. In situ rate measurements of microbial processes regulating nitrogen cycles in land water interfaces, and organic matter character and concentration, together with geochemical indicators, will provide valuable data on the rate of evolution of nitrogen land water buffer zones. These data are essential for understanding the consequences of deglaciation on greenhouse gas emissions (N2O, CO2, CH4) and the mechanisms by which nitrogen buffering capacity develop over time.